1. Field of the Invention
The present invention relates to a fluorescent lamp having a multi-ringed structure or arrangement having a rectangular shape and a lighting apparatus utilizing such a fluorescent lamp.
2. Related Art
In recent years, in addition to conventional fluorescent lamps having straight structure or ringed structure, high luminance fluorescent lamps having a relatively smaller tube diameter and a high load on a tube wall have been placed on many kinds of markets. There exists, as a certain kind of lamp of these high luminance fluorescent lamps, a double circular fluorescent lamp in which two circular bulbs having the same diameter that have been formed by subjecting a straight glass tube to a bending process are placed apart from each other in a perpendicular direction to a diametrical direction of the bulb and then subjecting the corresponding end portions of these bulbs to a bridge connection process to form a single discharge path, thus providing the fluorescent tube, i.e., the discharge path having a length about twice as long as the conventional circular fluorescent lamp (as described in Japanese Patent Provisional (KOKAI) Publication No. 2001-243914).
However, because the above-described ringed fluorescent lamp has a circular shape, it is necessary to carry out a bending process after heating the bulb to soften, thus leading to deteriorated properties of a phosphor layer and the glass bulb caused by heat. Initial luminous flux and luminous flux maintenance factor are therefore inferior to those of the straight structure fluorescent lamp.
In view of these matters, the inventors of the subject application have previously developed a double ringed fluorescent lamp having a rectangular shape. The double ringed fluorescent lamp has a structure that two, large and small ring glass bulbs, in which straight tube portions and bent portions are connected alternately into a rectangular shape, are placed concentrically on the same plane, the corresponding end portions of these ring glass bulbs are connected to each other through a bridge connection process to thereby form a single discharge path, and a base is mounted so as to straddle a space between the corresponding other end portions of the ring glass bulbs.
Such a newly developed double ringed fluorescent lamp has the glass bulbs with a rectangular shape, for example, square shape, thus providing an excellent fittability for a lighting apparatus having a rectangular shape and permitting extension of the length of the discharge path. Further, there is no need to apply a heat bending treatment to the straight tube portions of the ring glass bulb, with the result that deterioration by heat can be restrained and a high luminous efficacy can be obtained, thus providing advantageous effects.
The above-mentioned double ringed fluorescent lamp having the rectangular shape has the following structural features by way of example. More specifically, the two large and small ring glass bulbs that are placed concentrically are provided with the bent portions so that the centers of radii of curvature of the both ring glass bulbs coincide with each other. Accordingly, the radius of curvature of the outer ring glass bulb is larger than that of the inner ring glass bulb. As a result, a gap between the two ring glass bulbs is identical to each other in any position of the straight tube portion and the bent portion. The double ringed florescent lamp therefore has a good external appearance.
However, the inventors of the subject application carried out extensive studies to further enhance luminescence power of the double ringed florescent lamp having the rectangular shape and obtained findings that the luminescence power of the double ringed florescent lamp could further be enhanced by changing or modifying a portion of the above-mentioned structure.
Incidentally, when carrying out a bridge connection process to manufacture the above-described double ringed fluorescent lamp in which the large and small ring bulbs are connected through a bridge connection portion to form a single discharge path, one ring glass bulb is usually shifted linearly in the axial direction of the bridge connection portion to be formed, relative to the other ring glass bulb. A smaller distance between the large and small ring glass bulbs leads to a good external appearance and a smaller length of the bridge connection portion, thus providing advantageous effects of improving a mechanical strength of the double ringed fluorescent lamp.
On the other hand, a smaller distance between the ring glass bulbs makes it difficult to ensure the shifting distance of the bulb in the axial direction of the bridge connection portion when forming the bridge connection portion. In view of this fact, it has been found to be preferable to shift the inner ring glass bulb relative to the outer ring glass bulb so as to bring the outer peripheral side surface of the former into contact with the inner peripheral side surface of the latter, in order to ensure the largest distance within which the inner ring bulb can be shifted relative to the outer ring bulb.
In the conventional fluorescent lamp, when the inner ring glass bulb 1B is shifted in the direction of an arrow shown with the solid line as shown in FIG. 7A, the straight tube portion 1aB thereof comes into contact with the straight tube portion 1aA of the outer ring glass bulb 1A. However, when application of external vibration causes the ring glass bulb 1B to deviate in the direction of an arrow shown with the dotted line after the shifting operation or in the direction of an arrow shown with the dashed line during the shifting operation, the bent portion 1cB of the inner ring glass bulb 1B comes into contact with the bent portion 1cA of the outer ring bulb 1A as shown in FIG. 7B, thus making it difficult to ensure the proper shifting distance. Because the shifting distance and the shifting direction when forming the bridge connection portion are restricted accordingly, a thick wall section is formed in the bridge connection portion to bring about strain, thus causing problems of deterioration of the mechanical strength.